"If you only screen early or if you only screen once, you will miss some," said Linda Chaudron, M.D., an assistant professor of psychiatry, pediatrics and obstetrics and gynaecology at the University of Rochester Medical Center who is leading a series of studies focusing on postpartum depression.

In a recent analysis of records from a paediatric clinic that uses a common postpartum questionnaire to screen mothers, Chaudron and the research group found that of women who scored high on a depression screening scale sometime in the postpartum year, 26 percent did not develop high symptom levels of postpartum depression until after three months and that 33 percent had high levels throughout the year.

Earlier this year, New Jersey Governor Jon S. Corzine signed legislation requiring health care professionals providing postnatal care to screen new mothers for postpartum depression, and requiring health care professionals to educate women and their families about the disorder. Health care providers in several other states have adopted similar screening programs.

"With the increased attention to screening, we ought to have a better idea about when to screen," Chaudron said. "There are a lot more women we need to be thinking about, identifying and helping get treatment." Paediatricians and other physicians who refer mothers for treatment of depression also can help track how mothers are doing, even if they are in treatment. "They should find out how the mother is doing and whether she is receiving the proper treatment," Chaudron said.

The study reported in Ambulatory Pediatrics was limited. The records covered only 49 mothers. But Chaudron said the report highlighted that women can develop new symptoms later in the year and can continue to have symptoms for many months.